The present invention relates generally to nuclear reactor fuel assemblies and more particularly to a grid, for the spacing of fuel rods, which has improved grid straps.
This patent application is related to the concomitantly filed patent application entitled "Nuclear Fuel Spacer Grid with Improved Outer Straps" invented by E. E. DeMario and R. P. Knott.
Nuclear fuel spacer grids are used to precisely maintain the spacing between fuel rods in a nuclear reactor core, prevent rod vibration, and provide lateral support for the fuel rods. Conventional designs of grids for nuclear reactor fuel assemblies include a multiplicity of interleaved grid straps of egg crate configuration designed to form cells which accept fuel rods. Slots are utilized to effect an interlocking engagement with adjacent straps. Each cell provides support to one fuel rod at a given axial location through the use of relatively resilient springs and relatively rigid dimples formed into the metal. Outer straps are attached together and peripherally enclose the grid straps to impart strength and rigidity to the grid. In order to minimize the lateral displacement of fuel rods and to improve the fuel characteristics of an assembly, a number of grids are spaced along the fuel assembly length.
In a pressurized water reactor (PWR), typically some of the grid's cells accept control rod guide thimbles instead of fuel rods. Each grid is held in place along the fuel assembly length by attachment to the guide thimbles.
In a boiling water reactor (BWR), typically a can surrounds the fuel rods and grids. Each grid is held in place along the fuel assembly length by a special locking rod. In one conventional arrangement, at a given grid vertical location in the fuel assembly there are four horizontally aligned grids in a square array surrounded by the can.
Designers are constantly seeking to improve the performance of grids. Areas of concern include reducing the pressure drop of the longitudinal coolant flow passing through the grid and coping with loss of spring force due to irradiation induced stress relaxation, as well as cushioning of fuel assemblies during transport. Items of particular importance to BWR fuel assemblies include cushioning the grid against seismic forces acting on the can and directing the relatively cooler water flow from along the walls of the can towards the relatively hotter flow through the interior cells of the grid.